The continuing development and improvement of renewable electrical generation facilities such as wind, solar, and energy storage, coupled with growing global emphasis on satisfying rising demands for electrical power using “green” facilities, has led to the increasing use of automated control systems to optimize active and reactive power control, and to maximize the efficient generation, transmission, and distribution of electrical power provided by renewable technologies. In addition, maintaining operational limits of generation, collection system, substation, and transmission equipment, and the ability to swiftly react to grid disturbances have become key initiatives to enforce grid reliability, thus further complementing the use of automated control.
For a variety of reasons, renewable sites tend to be constructed in “clusters,” and are frequently located at distances geographically remote from the industrial and urban centers for which electrical power is required. In many cases the sites will incorporate different equipment manufacturers and different technologies, such as solar or wind electrical generation, or storage using batteries or other storage technologies. In addition, renewable sites are commonly restricted in various ways by Interconnect Agreements (IAs), which establish parameters for power being delivered to the grid at points of interconnect (POIs) between the clusters and the grid. IAs frequently control such parameters, also known as “set points,” as voltage, active and reactive power, primary source (site), and transmission limits. Some IAs also include a schedule of different voltages to be delivered at different times, according to the schedule. Because “green” power generation tends to occur in rural areas, power is frequently delivered to multiple off-takers—municipalities and “green” companies—each of whom may have somewhat different requirements, and whose requirements must be incorporated into the delivered product.
As new “green” power generation facilities are built, they must be incorporated into IAs, which may place limiting requirements on them, and must be integrated with existing facilities without changing the set points established by the relevant IA for power being delivered to the POI with the grid. For instance, when a new wind site is being added to an existing wind farm, the AI may require that the primary source of power be the original (older) site, with the new site being secondary in terms of power being delivered to the POI, at least until the stability and reliability of the enlarged system has been established.
In conjunction with off-taker and transmission operator demands and requirements, manual regulation of power generation and reactive power (typically voltage control) produced at such sites can prove difficult.
Reactive Power Control
Renewable sites are commonly required to control voltage (as required by the relevant IA), and typically this is done via reactive power controllers. A reactive power controller enables each site to produce or consume reactive power, thus achieving the desired power factor, voltage, or reactive power setpoint at the requisite location such as the POI. However, when additional renewables sites are added to an existing system, an unstable condition may develop whereby the reactive power flows between the sites, rather than supporting the grid in concert as originally intended. Further consequences such as protective shutdowns or failure to reach the mandated setpoint can result if properly coordinated reactive power control of the multiple renewables sites is missing.
Generation Control (Curtailment, Frequency Response, etc.)
The transmission of electrical energy from generation sites to areas where it will be used is often limited by the capacity of the transmission lines, a phenomenon which is referred to as “congestion.” Congestion occurs on electric transmission facilities when actual or scheduled flows of electricity across a line or piece of equipment are restricted below desired levels. These restrictions may be imposed either by the physical or electrical capacity of the line, or by operational restrictions created and enforced to protect the security and reliability of the grid. The term “transmission constraint” can refer to a piece of equipment that restricts power flows, to an operational limit imposed to protect reliability, or to a lack of adequate transmission capacity to deliver potential sources of generation without violating reliability requirements. Because power purchasers typically try to buy the least expensive energy available, when transmission constraints limit the amount of energy that can be delivered into the desired load center or exported from a generation-rich area, these constraints (and associated congestions) impose real economic costs upon energy consumers. In the instances where transmission constraints are so severe that they limit energy deliverability relative to consumers' electricity demand, such constraints can compromise grid reliability.
In cases in which more electricity is generated at renewables' sites than can be delivered using available transmission lines, it may be necessary to reduce electrical generation, an action otherwise known as “curtailment.” Renewables curtailment typically occurs when there is excess electric production in an area and there is insufficient transmission capacity to move that electricity to demand centers. While other kinds of power plants typically reduce their output before renewables' plants do, given that the fuel costs and other operating costs of those plants are higher than those of a renewable plant, in some cases renewable plants may also be called on to reduce their output. Almost all renewable plants can curtail their output utilizing numerous methods, yet accomplishing the same goal.
There are other reasons why electrical generation may be curtailed, including the incidence of relatively high winds during times of minimum or low load, little to no cloud cover, and market factors such as relative costs of electricity. Factors related to the curtailment of wind power are identified and discussed in a report, Wind Energy curtailment Case Studies May 2008-May 2009, published by the United States National Renewable Energy Laboratory (NREL), NREL/SR-550-46716 (October 2009), the contents of which are hereby incorporated in their entirety.
So long as curtailment remains a primary means for regulating the delivery of electrical power to a power grid, inefficiencies will be inherent in the system. However, when curtailment is in effect amongst a group of renewable energy sites, or amongst two or more sites, inefficiencies may be reduced or minimized by appropriate balancing of the amount of active power being produced by each site or facility to maximize the efficiency and output of multiple sites while being curtailed.
Maximizing the efficiency and balancing the flow of reactive power during curtailment also needs to be addressed. Balancing is normally accomplished by using reactors—inductance devices—to consume reactive power (−VARs), and capacitors to create reactive power (+VARs). In addition, some wind turbine manufacturers are making internal circuitry available in the turbine unit that can apply capacitance or inductance when the turbine is not producing power. Since such circuitry is present and used during normal energy production to adjust the active and reactive power output, it can be made available within a local cluster of wind turbines, for example, to adjust power factor as necessary to achieve desired set points for the wind farm. Since the amount of reactive power varies with the total power being generated, the ability to make adjustments in this parameter without adding additional hardware may provide an overall benefit to the wind farm operator.
Alternating current (AC) electrical frequency is the number of cycles per second (Hz) with the United States standard for grid frequency being 60 Hz. Maintaining proper frequency is paramount for grid reliability and current trends indicate that the ability to react to frequency deviations—either negative or positive—will become customary in the near future for all renewables sites. A negative response entails the curtailment of power, which in turn, would pull the excessive frequency down to 60 Hz which is essentially is a hasty curtailment. On the other hand, experience has shown that a circumstance can arise whereby any number of neighboring renewable sites is curtailed and a nearly instantaneous injection of generation up to the total potential generation is necessitated for frequency support (positive—to increase grid frequency to 60 Hz). In this scenario, a rapid reaction (releasing curtailment) is vital, and is not feasible through manual curtailment intervention.
Another closely related problem with renewable sites occurs when voltage generated by wind turbines drops below nominal voltage for extended periods of time. Standard operational voltages range from between 90% to 110% of the nominal voltage being produced. Whenever the operational voltage drops below 90% of the standard operating range for a short period of time, it is considered a “low voltage excursion.” This is a wind turbine issue that may affect an entire cluster. Under some circumstances, the low voltage may be sustained for longer than a short period of time. A Sustained Low Voltage Excursion occurs when the voltage is between 75% and 90% of nominal for a period of time of about 10 minutes, or 610 seconds. Low voltage excursions are costly, and can result is substantial revenue loss over time. Thus, a purpose of the invention is to eliminate such sustained low voltage excursions at wind sites by raising turbine voltage which can be done by increasing (producing) reactive power through reactive power controllers.
In the prior art, modifications to the output of individual turbines and wind farms were made manually, as conditions changed or as off-takers provided new or different parameters for the supply of power. One consequence of having to make changes manually was that set points were often established below desired levels in order to ensure that rapidly changing wind conditions would not increase power output above levels that were acceptable to off-takers before manual corrections could be applied. Attempts to resolve or improve the problem of making manual changes were made by substituting a programmable logic controller (PLC) to integrate real-time control logic to monitor the system and automatically make necessary adjustments when set points were changed or when output parameters changed as a result of increased winds or variances in the load. While using real-time logic control substantially improved response time for individual turbine units, the system of interconnected clusters experienced inefficiencies resulting from the fact that changes made in the output of single units affected overall system stability and caused unwanted side effects, such as the simultaneous creation of reactive power in one unit or farm and the consumption of that reactive power in another. Although power delivered to the POI was within off-taker set points, reactive power within the interconnected system would flow from one unit to another, and caused economic losses due to that inefficiency.
The realities of neighboring renewable sites and coordinating multiple site reactive power coordination, voltage schedules, long transmission lines, grid disturbances, frequency support, and balancing curtailment can prove difficult for manual regulation of power generation and reactive power. Therefore, this invention is directed to the automation of monitoring and control for reactive power and generation due to curtailment resulting from marketing or price balancing, transmission constraints and limits, imposed requirements such as those from off-takers, transmission operators and reliability coordinators, and most importantly grid reliability.
The invention also provides a means for master override control. By enabling multiple sites to communicate with one another to automatically control and monitor a single point of interconnect, the system operator can remotely control a site consisting of two or more individual sites to maximize efficiency and output while remaining in compliance with the various requirements such as voltage schedules or reactive power ranges. Additionally, appropriate balancing from multiple sources enables the operator to reduce response time for the marketing group, ISOs, and for the operator to curtail and release the renewables' sites, and provides continuous control and monitoring of multiple sites at a single point of interconnect. Finally, third parties such as off-takers can now directly control generation and reactive power on an as needed basis.